Can’t help with baylage but I was like you with greying hair and had it professionally coloured for years, eventually became 100%grey.
After a lot of nagging from a friend (who is a box dye fan and looks fabulous) during first lockdown I resorted to box dye. Tried a few brands and shades. My conclusions, for what they’re worth, are:
the expensive brands, where you have an online consultation first, are no better than the cheap ones
the colours are always a bit darker than on the box when first applied
even the cool browns have warm highlights in them
it still takes me over an hour just to apply, my friend does the back for me
deciding how long to leave on - both roots and then all over - is unique to you and a matter of trial and error
I have an impressive collection of old stained towels to protect table and floor with before I start ( I am such a clumsy cow)
even a semi permanent (I liked nice’n’ easy) will end up as permanent because the colour never fully washes out, but it might be a good place to start as you build up confidence
my hair quickly returned to good glossy condition after I quit the salon colouring,
the salon products are very strong and damaging, so they end up selling you various treatments as well
the cheap dyes are gentler, they must have less of the damaging chemicals in them, and just as effective - my favourite is Superdrug performance 5.00, but now I understand the number system I do ring the changes a little
the conditioner provided with the dye is marvellous and with my shorter hair lasts for a few washes
I use cheap colour protect shampoo and conditioner like L’Oreal
it now takes me 15 months to spend on colouring what I previously spent in each salon visit - and that’s excluding the cost of cut and blow and before the treatments (Olaplex and such like) were added on
Hope this helps.